Molecules are stored in ice just before star and planet formation

April 11, 2022

Astronomers at the Max Planck Institute for Extraterrestrial Physics have found evidence that just before star formation, in the central region of a pre-stellar cloud, practically all heavy molecules freeze out on top of dust grains. The ALMA observations of the L1544 cloud in the constellation Taurus showed not only a central concentration of dust grains, but also revealed that molecules containing nitrogen as well those containing carbon, oxygen and all elements heavier than helium, are stored in thick icy mantles around the dust grains. These icy mantles are rich in water and organic molecules, precursors of pre-biotic molecules. The abundances are similar to those observed in leftover objects from the formation of our Solar System.

How do planets and stars form? This is one of the central questions in modern astrophysics. While the broad strokes are clear – a cold molecular cloud collapses under its own gravity, an accretion disk forms, and at its centre a proto-star – the devil is in the detail. One crucial step is the so-called pre-stellar core phase, when the interstellar gas cloud is contracting while flattening (on its way toward the formation of a protoplanetary disk), but before the gravitational pull produces a central proto-star.  

Astronomers at the Max Planck Institute for Extraterrestrial Physics have now observed such a pre-stellar core, called L1544 in the constellation Taurus, in unprecedented resolution with the ALMA radio telescopes. “Studies of pre-stellar cores in nearby clouds have provided clues on their physical and chemical structure, but it was still unclear what happens at the very centre,” points out Paola Caselli, lead author of the paper now published in the Astrophysical Journal. “Now, we can study structures in the central 2000 Astronomical Units (AU), where a future stellar system will form.” For comparison: Neptune, the outermost known planet in our home Solar system, is at a distance of 30 AU from the Sun, while the Kuiper belt and the so-called scattered disk, where short term comets and other icy bodies reside, extend to about 200 AU.

The observations included both continuum emission of dust grains in this pre-stellar core and spectral line observations of deuterated ammonia, i.e., a molecule made up of nitrogen and hydrogen, where one hydrogen atom is substituted by a deuterium atom (NH2D). While the dust continuum emission revealed a compact central region with a mass of about 1/6 the mass of our Sun, the molecular line analysis was the real surprise. For the first time, the observations provided evidence of almost complete freeze-out: practically all (99.99%) molecules and atoms heavier that helium disappear from the gas and condense on top of dust grains in the central 2000 AU.

“This suggests a “complete-depletion zone” in agreement with astrochemical pre-stellar core model predictions,” explains Olli Sipilä, who carried out the theoretical modelling. The state-of-the-art chemical model actually predicts that the freeze-out starts already at 7000 AU and radiative transfer effects cause the emission of some molecules to appear centrally concentrated. “This has prevented the freeze-out to be detected in previous observations, where the centre could not be resolved,” he adds.

The dust grains in such a pre-stellar core thus become surrounded by thick icy mantles, rich in water and organic molecules, which form the building blocks for future planets. A recent study of the comet 67P/CG has indeed shown that it contains molecules with relative abundances similar to pre-stellar cores and young star forming regions.

“We were able to demonstrate that pre-stellar molecules are “stored in ice” before the formation of a stellar system similar to our own,” explains Jaime Pineda, second author of the paper. Some of this pre-stellar ice, especially icy pebbles in the outer part of the disk, may even survive to later stages of planet formation, preserving the chemical signature of these primordial phases just before the switch on of a new star. “Icy bodies now present in the outskirts of our Solar System may indeed contain the “frozen” chemical history of our pre-Solar core, the cloud out of which all we see today in our Solar System (including us) originated”, concludes Paola Caselli. “As some of the icy pebbles in the young Solar System are known to have drifted toward the Terrestrial planet formation zone, the icy grains in the centre of our pre-Solar core may have even contributed to volatile molecules, including water and organics, in our Earth, i.e., they may have provided precious ingredients for the origin of life on our planet.”

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